1. Field of the Invention
This invention relates generally to an automatic transmission for a motor vehicle, and, more particularly, to a hydraulic pump for the transmission driven by an electric motor and engine.
2. Description of the Prior Art
Current automatic transmissions having hydraulically actuated clutches and brakes for controlling the gearing use a hydraulic pump to pressurize and pump fluid to the control elements. Typically the pump is driven directly by an engine via a mechanical coupling.
Such pumps can be broadly divided into fixed displacement pumps (FDP) and variable displacement pumps (VDP). Fixed displacement pumps deliver a constant volume of fluid per revolution and the total volume per unit of time is directly proportional to its speed. Fixed displacement pumps produce a flow rate that is set at minimum engine speed based on a required system flow rate. As a result, at higher speeds, excess fluid flow must be return to an oil sump or recirculated to the pump inlet. The excess flow decreases the operating efficiency of the transmission.
The fluid displacement per revolution of a variable displacement pump can be adjusted to deliver a variable flow rate, i.e., the volume of fluid per unit of time, e.g. liters per minute, at a constant speed.
Variable displacement pumps typically used in automotive applications are variable displacement vane pumps, whose displacement is adjusted by a control system as fluid flow requirements are met. Excess flow generated by the pump is utilized to actuate the pump's control, which adjusts the eccentricity of a control ring relative to the vanes.
Such control mechanisms have limited capability to adjust flow rate. This limitation is realized at maximum transmission speed, at which the eccentricity cannot be further decreased, yet the pump is still providing flow in excess of the transmission system's requirements. Excess flow, under these conditions is exhausted to sump, thereby adversely affecting the pump's mechanical efficiency and vehicle fuel economy.
Changes in the flow rate due to transient conditions, such as gear or pressure changes, can occur within milliseconds, but the response delay of a VDP displacement adjusting mechanism typically cannot match the change in flow rate demand. As result, VDPs must be oversized to handle transient flow demands.
An ideal pumping system using an electric motor with variable speed control and a pump is not practical in an automatic transmission that operates over a wide range of operating conditions including cold start-ups that require high torque. The high torques in cold temperature operation would require high power current supply.
An ideal pump, i.e., a pump consuming the minimum energy, should have infinitely variable flow rate depending on system flow demand, which is defined as the instantaneous fluid flow rate that is required to satisfy hydraulic system functions such as, but not limited to cooling, clutch actuations, lubrication, leakages. System flow demand can be further divided into steady state and transient demands. Flow demand generally depends on fluid temperature, viscosity, circuit pressure and other operating conditions. Transmission system flow demand is independent of pump flow delivery.